In the cellular system known as E-UTRAN, Evolved Universal Terrestrial Radio Access Network, also known as the Long Term Evolution system, LTE, the downlink transmissions (i.e. transmissions from the controlling node of a cell to users in the cell) are based on Orthogonal Frequency Division Multiplexing, OFDM, with OFDM symbols which can extend over a number of subcarriers and which have a certain extension in time as well.
Due to this, the E-UTRAN physical downlink resource can be seen as a time-frequency grid, comprising a number of resource elements, with each resource element corresponding to one OFDM subcarrier in one OFDM symbol interval.
In time, the E-UTRAN downlink transmissions are organized into so called radio frames, each of which comprises ten so called subframes with an extension in time of 1 ms, so that an E-UTRAN radio frame has a total extension in time of 10 ms.
In E-UTRAN systems, so called L1/L2 control signalling is used for transmitting downlink scheduling assignments, which are required for the users (“terminals”) to properly receive, demodulate and decode downlink data, as well as uplink scheduling grants informing the terminals about resources and transport format for the uplink transmissions, together with hybrid-ARQ acknowledgements in response to uplink data transmission.
In E-UTRAN, the downlink L1/L2 control channels are mapped to the first 1-3 OFDM symbols within a subframe. Thus, each E-UTRAN subframe can be said to be divided into a control region and a data region, with the control region being first in time.
The size of the E-UTRAN control region is always equal to an integer number of OFDM symbols (1, 2 or 3 OFDM symbols can be used for control signalling) and can be varied per subframe, which maximizes the spectral efficiency as the control signalling overhead can be adjusted to match the instantaneous traffic situation.
The location of the control signalling at the beginning of the subframe is advantageous, as it enables a terminal to decode the downlink scheduling assignment (DL-SCH) prior to the end of the subframe. Decoding of the DL-SCH can thus begin directly after the end of the subframe, without having to wait for the decoding of the L1/L2 control information, which minimizes the delay in the DL-SCH decoding and thus the overall downlink transmission delay.
The E-UTRAN downlink L1/L2 control signalling consists of three different physical channel types:                PCFICH, the Physical Control Format Indicator Channel, which is used to inform the terminal of the number of OFDM symbols (1, 2, or 3) that are used for L1/l2 control signalling in the current subframe. At present, there is only one PCFICH in a cell.        PDCCH, the Physical Downlink Control Channel, which is used to carry downlink scheduling assignments and uplink scheduling grants. In addition, it may also be used for power control of a group of terminals. Typically, there are multiple PDCCHs in a cell.        PHICH, the Physical Hybrid-ARQ Indicator Channel, which is used to transmit ACK/NACK in response to reception of UL-SCH transmissions. Typically, there are multiple PHICHs in a cell.        
A closer description of the PCFICH and PHICH control channels is as follows:
PCFICH—the Physical Control Format Indicator Channel
The PCFICH is used to indicate the number of OFDM symbols used for L1/L2 control signalling in the current subframe, or, equivalently, where in the subframe the data region starts. Reception of the PCFICH is thus essential to correct operation of the system. If the PCFICH is incorrectly decoded, the terminal will neither know where to find the control channels, nor where the data region starts, and will therefore lose any uplink scheduling grants transmitted, as well as any DL-SCH data transmission intended for the terminal.
At present, two bits of information, corresponding to a control region size of 1, 2, or 3 OFDM symbols, are coded into a 32-bit long sequence using a so called rate- 1/16 simplex code. The coded bits are scrambled, QPSK-modulated, and mapped to 16 E-UTRAN OFDM resource elements. To be compatible with the different E-UTRAN transmit diversity schemes, which are specified in groups of 4 symbols, the 16 resource elements are grouped into 4 groups of 4 elements each. Such a group of 4 resource elements is sometimes referred to as a mini-CCE (Control Channel Element), also known as a resource-element group.
Frequency diversity is important for reliable PHICH reception. Therefore, the PCFICH is mapped to 4 mini-CCEs which are well separated in frequency. In the current 3GPP (3rd Generation Partnership Project) specifications, this is obtained by dividing the overall downlink system bandwidth into four equally-sized quarters with one mini-CCE in each quarter, so that the mini-CCEs used for PCFICH are equally spaced in frequency.
PHICH—the Physical Hybrid-ARQ Indicator Channel
The PHICH is used for transmitting hybrid-ARQ acknowledgements in response to UL-SCH transmission. There is one PHCIH for each terminal which expects an acknowledgement in the subframe.
Each PHICH carries one bit which is repeated three times, modulated, spread with a spreading factor of four, and then mapped to 3 mini-CCEs. Multiple PHICHs form a so called PHICH group, and the PHICHs within a PHICH group are code-multiplexed using different orthogonal spreading sequences, and share the same set of resource elements.
In similarity to the PCFICH, frequency diversity is important for the PHICH. At present, the exact mapping of PCFICH in E-UTRAN has not been decided in 3GPP, but ideally the 3 mini-CCEs used for a PHICH should be spread over the full system bandwidth.
Typically, the PHICH is transmitted in the first OFDM symbol only. However, in some propagation environments, this would unnecessarily restrict the PHICH coverage. To alleviate this, it is possible to configure a PHICH duration of three OFDM symbols, in which case the control region will be three OFDM symbols long in all subframes.
At present, there is no specification in E-UTRAN for the PHICH mapping. However, to use the same approach as for the PCFICH, i.e. to space the mapping of the three PHICH mini-CCEs equally over the system bandwidth might cause problems, since this could result in the PHCIH being mapped to the same set of resource elements as the PCFICH.